Showing papers on "Stellar nucleosynthesis published in 1993"
TL;DR: The search is now on for other surviving stellar condensates, such as nitrides and oxides as discussed by the authors, in the solar nebula and early Solar System, and the search is also on for surviving stellar nucleosynthesis and condensation processes near carbon stars.
Abstract: Primitive meteorites contain grains of silicon carbide, graphite and diamond formed outside the Solar System and probably before its birth The isotopic compositions of these grains provide a record of stellar nucleosynthesis and of condensation processes near carbon stars; the fact of their survival places constraints on conditions in the solar nebula and early Solar System The search is now on for other surviving stellar condensates, such as nitrides and oxides
259 citations
TL;DR: In this paper, the nucleosynthesis resulting from integrating the yields of these models over plausible initial stellar mass distributions is found to be in excellent agreement with the observed solar abundances of virtually all the intermediate mass isotopes (16 ≤ A ≤ 32) if, and only if, the rate of the 12C(α, γ)16O reaction is taken to be 1.7 ± 0.5 times that given by Caughlan and Fowler.
253 citations
TL;DR: In this article, the authors consider various models for the chemical evolution of the Galaxy to test the feasibility for the destruction of D without the overproduction of $3He$ and overall metallicity and discuss the effect of parameters that govern the initial mass function and of surviving fractions of $ 3He$ in stars between one and three solar masses.
Abstract: The determined abundances of primordial $^4He$ and $^7Li$ provide a basis with which to test the standard model of big bang nucleosynthesis in conjunction with the other two light element isotopes $D$ and $^3He$, also produced in the big bang. Overall, consistency in the standard big bang nucleosynthesis model is best achieved for a baryon-to-photon ratio of typically $3 \times 10^{-10}$ for which the primordial value of $D$ is five times greater than the present observed abundance and about three times greater than the pre-solar value. We consider various models for the chemical evolution of the Galaxy to test the feasibility for the destruction of D without the overproduction of $^3He$ and overall metallicity. Models which are capable of achieving this goal include ones with a star formation rate proportional to the gas mass fraction or an exponentially decreasing star formation rate. We discuss the effect of parameters that govern the initial mass function and of surviving fractions of $^3He$ in stars between one and three solar masses.
48 citations
TL;DR: In this paper, the authors discuss the importance of Population II Li-6 as a diagnostic for models of primordial nucleosynthesis, cosmic-ray nucleosyntheses in the early Galaxy, and the structure and evolution of metal-poor solar-type stars.
Abstract: We discuss the importance of Population II Li-6 as a diagnostic for models of primordial nucleosynthesis, cosmic-ray nucleosyntheses in the early Galaxy, and the structure and evolution of metal-poor solar-type stars. The observation of Li-6 in the subdwarf HD 84937 is shown to be consistent with the existing Population II LiBeB data within the context of a simple three-component model: (1) standard big bang nucleosynthesis, (2) Population II cosmic-ray nucleosynthesis, (3) standard (nonrotating) stellar LiBeB depletion. If this interpretation is correct, we predict a potentially detectable boron abundance for this star: about 2 x 10 exp -12. Subsequent Population II LiBeB observations, and in particular further observations of Population II Li-6, are shown to be crucial to our understanding of the primordial and early galactic creation and destruction mechanisms for light elements.
45 citations
TL;DR: In this paper, the authors considered the evolution of the light elements (Li, Be and $B$) incorporating the effects of their production by both neutrino process and cosmic-ray nucleosynthesis.
Abstract: We consider the evolution of the light elements ($Li, Be$ and $B$) incorporating the effects of their production by both neutrino process and cosmic-ray nucleosynthesis. We test the viability of the neutrino process to resolve the long standing problem of the $^{11}$B/$^{10}$B isotopic ratio which amounts to 4 at the time of the formation of the solar system. This hypothesis may be ultimately constrained by the $B/Be$ ratio observed in halo stars. Though we are able to obtain a solar isotopic ratio $^{11}B/^{10}B \simeq 4$, the current paucity of data at low metallicity prevents us from making a definitive conclusion regarding the resolution of this problem. We show however, that neutrino process nucleosynthesis leads to a relatively model independent prediction that the $B/Be$ elemental ratio is large ($>$ 50) at low metallicities ($[Fe/H] < -3.0$), if $Be$ is produced as a secondary element (as is the case in the conventional scenario of galactic cosmic-ray nucleosynthesis).
31 citations
TL;DR: In this paper, a simulation of the expected emissivity profile as a function of Galactic longitude is presented, taking into account plausible source distributions and a realistic spiral pattern for our Galaxy.
Abstract: Recent models of stellar nucleosynthesis suggest that massive stars are probably the main contributors of radioactive 26 Al in the Galaxy. In that case the angular distribution of the corresponding 1.8 MeV emission should trace, to some extent, the Galactic spiral arms, presumed site of massive star formation. Numerical simulations of the expected emissivity profile as a function of Galactic longitude are presented, taking into account plausible source distributions and a realistic spiral pattern for our Galaxy. The recently reported observations of the COMPTEL instrument aboard the Compton Observatory are discussed in light of those results
26 citations
TL;DR: In this article, a detailed study of inhomogeneous Big Bang nucleosynthesis is presented, where nuclear reactions are coupled to all significant fluctuation dissipation processes, including neutrino heat transport, baryon diffusion, photon diffusive heat transport and hydrodynamic expansion with photon-electron Thomson drag.
Abstract: We present a detailed study of inhomogeneous Big Bang nucleosynthesis where, for the first time, nuclear reactions are coupled to all significant fluctuation dissipation processes. Theses processes include neutrino heat transport, baryon diffusion, photon diffusive heat transport, and hydrodynamic expansion with photon-electron Thomson drag. Light element abundance yields are presented for broad ranges of initial amplitudes and length scales for spherically condensed fluctuations. The $^2$H, $^3$He, $^4$He, and $^7$Li nucleosynthesis yields are found to be inconsistent with observationally inferred primordial abundances for all but very narrow ranges of fluctuation characteristics. Rapid hydrodynamic expansion of fluctuations late in the nucleosynthesis epoch results in significant destruction of $^7$Li ($^7$Be) only if the baryonic conytribution to the closure density ($\Omega_b$) is less than or comparable to the upper limit on this quantity from homogeneous Big Bang nucleosynthesis. This implies that $^7$Li overproduction will peclude an increase on the upper limit for $\Omega_b$ for any inhomogeneous nucleosynthesis scenarios employing spherically condensed fluctuations.
21 citations
TL;DR: In this article, the cross sections of the reactions 34 S(α, γ) 38 Ar, 34 S (α, n) 37 Ar and 34 S p 37 Cl have been measured over bombarding-energy ranges of 2.79-5.87 MeV, 5.26-9.88 MeV and 5.70-10.33 MeV respectively.
15 citations
01 Jan 1993
TL;DR: In this paper, a multiphase model for the evolution of elliptical galaxies is presented, where diffuse gas, molecular clouds, stars and remnants are taken into account, and the occurrence of winds driven by supernovae is considered, allowing for further star formation from the restored gas.
Abstract: We present a multiphase model for the evolution of elliptical galaxies. Diffuse gas, molecular clouds, stars and remnants are taken into account. Cloud--cloud collisions and stimulated processes are the main causes of star formation. The occurrence of winds driven by Supernovae is considered, and the evolution of the system is computed also after the first wind, allowing for further star formation from the restored gas. The evolution of the abundances of 15 elements or isotopes is followed with detailed stellar nucleosynthesis. Stellar lifetimes are taken into account and a new IMF has been adopted. The gas removal due to the Supernovae explosions depends on the galactic mass and the presence of dark matter; the subsequent wind episodes are crucial to the intergalactic gas enrichment. Good agreement is obtained for current SNs rates, Star Formation Rate and gas masses when compared to the available data.
12 citations
Journal Article•
TL;DR: In this paper, the observed abundances of Cu and Zn in stars of different metallicities are compared with chemical evolution models for the Galaxy adopting current stellar nucleosynthesis results.
Abstract: The observed abundances of Cu and Zn in stars of different metallicities are compared with chemical evolution models for the Galaxy adopting current stellar nucleosynthesis results. Cu and Zn are both produced by s-processing in massive stars (weak component) and by s-processing in low mass stars (main component) as well as by explosive nucleosynthesis in supernovae. The most important contributions to Cu and Zn are those from supernovae and the weak component, while the contribution from the main component is negligible. The stellar yields for the weak component are now fairly well established, whereas those for the explosive nucleosynthesis are still affected by large uncertainties
11 citations
TL;DR: In this article, the most abundant products of stellar nucleosynthesis based on Bernal's models of dense liquid drops are related to the relevant nuclear binding energy data and shell models.
Abstract: Models of the bond structure of the most abundant products of stellar nucleosynthesis based on Bernal's models of dense liquid drops are related to the relevant nuclear binding energy data and shell models. Good agreement is obtained when alpha particles are considered to be the densely packed hard spheres of Bernal's models.
Journal Article•
TL;DR: In this paper, a quantitative estimate for heavy-elements enrichment in the early Galaxy that are due to the s-process nucleosynthesis in massive stars is made based on a realistic calculation of sprocess yields from massive stars with varied initial composition.
Abstract: A quantitative estimate is made for heavy-elements enrichment in the early Galaxy that are due to the s-process nucleosynthesis in massive stars. It is based on a realistic calculation of s-process yields from massive stars with varied initial composition, and a simple one-zone model of chemical evolution of the Galactic halo. In particular, we compare the predicted time-variations of Cu, Sr, Y and Zr abundances with those implied from observation. Unlike the others, the present work does not attempt to fit the observational data by parametrized models for various nucleosynthesis processes. Instead, we intend to set the limit to the contributions just from the s-process in massive stars by utilizing results of realistic nucleosynthesis calculation
TL;DR: In this article, the authors have sorted out the complex chain of stellar reactions responsible for the observed abundances of the isotopes, with remarkable success in all the most critical reactions but one: 12C(α,γ)16O.
Abstract: According to the standard model of the Big Bang, the only elements created during that event were hydrogen, helium and some lithium; the rest were brewed in the stars. Over the past few decades nuclear astrophysicists have sorted out the complex chain of stellar reactions responsible for the observed abundances of the isotopes, with remarkable success in all the most critical reactions but one: 12C(α,γ)16O. This reaction is a key stepping‐stone in the synthesis of heavier elements, linking the stage that burns helium to produce carbon and oxygen with the later stages fueled by those elements. If helium burning yields more or less oxygen or carbon, it is predicted that the later stages will be strongly affected, and so will the ultimate fate of massive stars.
Journal Article•
TL;DR: In this article, the CCD spectra for a sample of 8 type I planetary nebulae in the LMC were obtained in the wavelength region λλ 3800-8200 A. Electronic temperatures, densities and resulting abundances of He, N, O, S and Ar were obtained, where in particular the uncertainties in the He and O abundances are discussed.
Abstract: CCD spectra for a sample of 8 type I planetary nebulae in the LMC were obtained in the wavelength region λλ 3800-8200 A. Electronic temperatures, densities and resulting abundances of He, N, O, S and Ar were obtained, where in particular the uncertainties in the He and O abundances are discussed. Using results from stellar nucleosynthesis we estimated from the S/O and Ar/O ratios that the relative number of type I supernovae in the LMC appear to be twice higher than in our Galaxy
TL;DR: In this article, three aspects of nucleosynthesis in massive and intermediate-mass stars during their early evolutionary phases are discussed, related to the CNO abundances in giant or supergiant stars, to the 26A1 yield from massive stars via stellar wind, and to the production of the s-process nuclei in massive stars.
Abstract: We discuss three aspects of the nucleosynthesis in massive and intermediate—mass stars during their early evolutionary phases. These are related to the CNO abundances in giant or supergiant stars, to the 26A1 yield from massive stars via stellar wind, and to the production of the s—process nuclei in massive stars.
TL;DR: In this article, two aspects of the relation between heavy-element nucleosynthesis and galactic chemical evolution are examined and a schematic model for the formation and evolution of the Galaxy based upon the multiple mergers of protogalactic clouds within the expanding and collapsing halo is presented.
TL;DR: In this paper, the astrophysical S-factor and reaction rates for the triple-alpha process were calculated in the direct-capture model and it was shown that the stellar carbon production is extremely sensitive to small variations in the N--N interaction.
Abstract: The astrophysical S--factor and reaction rates for the triple--alpha process are calculated in the direct--capture model. It is shown that the stellar carbon production is extremely sensitive to small variations in the N--N interaction.
TL;DR: The possibilities and problems of studying nucleosynthesis and chemical evolution in the Galaxy by determining stellar abundances are illustrated in three examples as discussed by the authors, i.e., abundances in solar-type disk dwarfs with various ages and kinematic properties.
Abstract: The possibilities and problems of studying nucleosynthesis and chemical evolution in the Galaxy by determining stellar abundances are illustrated in three examples-the abundances in solar-type disk dwarfs with various ages and kinematic properties, the light element abundances in Population II dwarfs, and the molecular abundances in circumstellar shells around carbon stars.
TL;DR: In this article, a review of primordial nucleosynthesis is presented, in particular the consequences of a recent Monte Carlo analysis of standard big bang nucleosynthetic models, and the limits imposed by it on the baryonic density of the universe.
Abstract: A review of the current status of primordial nucleosynthesis is presented. In particular, the consequences of a recent Monte Carlo analysis of standard big bang nucleosynthesis are discussed. Such a study addresses the validity of the standard model, and the limits imposed by it on the baryonic density of the Universe. Inhomogeneous primordial nucleosynthesis models are reviewed in the light of the recent observations of beryllium in very metal poor halo stars. Beryllium production in the very early galaxy by a new mechanism, namely neutrino nucleosynthesis, is also discussed.